AC induction motors are widely used in a variety of industrial and residential applications. In general, this type of motor includes a laminated magnetic core mounted to a drive shaft. The laminated magnetic core may be fabricated from a plurality of laminated magnetic discs, or from a plurality of arc-like core segments. The laminated magnetic core includes a plurality of longitudinal slots into which bars of electrically conductive metal are fit. The ends of the bars extend beyond either end of the laminated magnetic core. An end-ring or end-cap at either end of the laminated magnetic core is used to mechanically and electrically join the ends of the rotor bars.
It will be appreciated that there are numerous techniques that may be used to fabricate the rotor assembly in general and the cap assembly in particular. Typically these techniques make trade-offs between several, often competing, factors that include (i) maximizing the electrical conductivity between the rotor bars; (ii) rotor weight; (iii) material cost; and (iv) fabrication/assembly cost and complexity. One approach that has been used to fabricate the rotor assembly is disclosed in U.S. Pat. No. 3,778,652. As described, a casting process is used to cast aluminum conductor bars in the slots within the laminated magnetic core. To improve the fit between the cast bars and the slots of the core, this patent discloses utilizing projections within the slots, thereby confining and minimizing the shrinkage of the cast bars to small regions. The casting process can be used to cast both the conductor bars and the end rings that electrically couple the bars together.
U.S. Pat. No. 4,064,410 discloses an alternate rotor fabrication process. As disclosed, rotor bars are first inserted into a laminated core such that end portions of each bar protrude beyond the end laminations at either end of the core. An end ring is then positioned over the shaft at either end of the core, the end rings having a plurality of channels on the inner ring surface that are designed to accept the ends of the rotor bars. Welding is then used to fuse the end portions of the rotor bars to the end rings, the welding process being carried out while applying an axial compression of the two rings toward one another.
U.S. Pat. No. 6,088,906 discloses several techniques for forming a joint between the rotor bars that extend beyond the laminated core and the end rings positioned at either end of the rotor assembly. In one of the disclosed techniques, the end rings are rotated about their rotational axes at high speed, and then simultaneously pushed into contact with the ends of the rotor bars. Frictional heating causes the ends of the rotor bars to fuse into the complementary surfaces of the rings. This frictional heating approach may be augmented by applying a high axial current to the end rings. Also disclosed is a technique in which a pulsed current generator is used to heat a foil of a brazing alloy to form a braze joint between the end rings and the ends of the rotor bars.
Japanese Patent Application No. 2003020929 (Publication No. 2004007949) discloses a rotor fabrication technique in which the end rings are formed of multiple, individual arc-like end ring pieces. The end ring pieces are positioned at the ends of the laminated core, between the rotor bars. A rotary tool is used to friction weld the end surfaces of the rotor bars to the end ring pieces.
While the prior art discloses a number of techniques that may be used to fabricate the rotor assembly of an electric motor, these techniques tend to provide inferior electrical conductivity, often while utilizing relatively bulky and weighty end rings. Accordingly, what is needed is a rotor assembly that can be easily fabricated while minimizing rotor weight and maximizing conductivity between rotor bars. The present invention provides such a rotor assembly.